Use ${MACHINE} for the kernel modeule ldscript

For consistancy with the kernel linker script also use ${MACHINE} for
finding the kernel module linker script. As we currently only use this
for amd64 and i386 this is a no-op, but I'm planning on using this with
arm64 where ${MACHINE} != ${MACHINE_ARCH}.

Reviewed by: markj, kib, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33841

(cherry picked from commit 2754373f9ac8e73f8fdbba9df26ac03e4864576e)